The present invention relates to an image forming apparatus comprising: an image carrier on which an electrostatic latent image is formed; means for forming a latent image on the image carrier; means for developing the latent image by abutting a developing roller on the image carrier; means for transferring a toner image developed on the image carrier onto an intermediate transfer medium; and a cleaner, abutting on the image carrier, for removing post-transfer residual toner, wherein the developing roller spaced apart from the image carrier during a development color switching operation.
An image forming apparatus includes, around the periphery of a photosensitive body serving as a rotary image carrier, a charging device to charge the photosensitive body uniformly along the rotational direction thereof, an exposing device to form an electrostatic latent image on the photosensitive body, a developing device to develop an electrostatic latent image, a transferring device to transfer a toner image on the photosensitive body onto a transferring member, a cleaner to clean the surface of the post-transfer photosensitive body, etc. An electrostatic latent image formed on the photosensitive body by the exposing device is developed by the developing device to form a toner image. The toner image thus formed is transferred onto a transferring member and residual toner on the post-transfer photosensitive body is removed from the photosensitive body by the cleaner.
In a case of a rotary-type full-color image forming apparatus that forms a full-color image, color toner images, formed on the photosensitive body by the respective developing devices for yellow Y, magenta M, cyan C, and black K, are sequentially transferred onto an intermediate transferring member to superpose colors. For this reason, the developing cartridges of respective colors of toner are detachably mounted to a rotary development unit, and each time an electrostatic latent image of one color is formed on the photosensitive body, the rotary development unit is driven by a development color switching operation to move the developing cartridge of the corresponding toner by rotation to a developing position for a developing operation to be performed. Such a configuration is disclosed in Japanese Patent Publication Nos., 2002-82532A, and 2003-66801A, for example.
In an image forming apparatus, an outer circumferential surface of the photosensitive body is repetitively scanned in the axial direction thereof (primary scanning direction) to form an electrostatic latent image thereon while the photosensitive body is uniformly rotated. An exposing beam from the exposing device is modulated by an image forming signal. Therefore, when the rotations of the photosensitive body are unstable to cause vibrations and irregularities, there occurs a problem that the image quality is deteriorated due to banding, which appears, for example, in the form of a streak in the secondary scanning direction.
One of the factors that make rotations of the photosensitive body unstable to cause vibrations and irregularities is, besides the configuration of a gear train in the driving mechanism, the presence of the cleaner that abut on the photosensitive body along the periphery thereof.
The cleaner is to clean residual toner on the post-transfer photosensitive body after a toner image formed on the photosensitive body is transferred onto a transferring member. In order to enhance the cleaning effect, tho cleaner is abutted on the outer circumferential surface of the photosensitive body at a constant pressure by a spring or the like. A lower abutting pressure naturally reduces the cleaning effect; however, a higher abutting pressure becomes a factor that generates irregular vibrations associated with frictional sliding between the outer circumferential surface of the photosensitive body and the cleaner. In addition, the angle at which the blade abuts on the outer circumferential surface of the photosensitive body, the position of the supporting point, at which the cleaner is supported, with respect to the tangential line to the abutting surface, etc. have an influence on the cleaning effect.
Further, in contrast to an image forming apparatus in which the developing roller constantly abuts on the photosensitive body, in an image forming apparatus in which the developing roller is retracted from the developing position on the photosensitive body during the development color switching, it is observed that irregularities in rate of the photosensitive body readily occur. For example, in a rotary-type color image forming apparatus provided with the rotary development unit, the rotary development unit is rotated when the development color switching operation is performed while each of the developing rollers is sequentially brought into contact with the photosensitive body and spaced apart therefrom. Due to the repetition of the abutment and retraction, the outer circumferential surface of the photosensitive body cannot be maintained in the uniform state, and the cleaner is no longer able to undergo frictional sliding smoothly, which in turn makes a rate of the photosensitive body unstable.